One form of linear actuator used in aerospace applications comprises a threaded screw shaft which is supported in such a manner as to be axially moveable whilst held against significant angular motion. A rotatable nut cooperates with the screw shaft in such a manner that rotation of the nut drives the screw shaft for axial movement. Conveniently, a ball-screw type coupling is provided between the nut and the screw shaft so as to reduce frictional losses within the actuator. The nut is conveniently arranged to be driven for rotation by an associated electrically powered motor.
Linear actuators are used in a wide range of aerospace related applications. For example they are used to drive thrust reverser components, variable area fan nozzle components, under carriage components and the like for movement between stowed and deployed positions.
Increasingly there is a desire to use electrically driven linear actuators in applications in which, traditionally, it has been usual to use hydraulically driven actuators. One such application is in adjusting the angle of the nose wheel of an aircraft in order to steer the aircraft whilst it is moving along the ground at relatively low speed.
A typical nose-wheel steering arrangement includes an axially translatable toothed rack, the teeth of which are cooperable with the teeth of a toothed pinion formation provided upon a support shaft associated with the nose-wheel. Axial movement of the rack drives the pinion, and hence the shaft, for angular movement, thus adjusting the orientation of the nose-wheel to effect steering. The rack is typically hydraulically driven.
As mentioned above, there is a desire to use electrically powered actuators in such applications. One way in which an electrically powered actuator can be used in controlling nose-wheel steering is to use an electric motor driven linear actuator to drive the rack of an arrangement of the type described hereinbefore for translating movement. For example, the actuator may comprise a rotatable, but axially fixed, threaded nut arranged to be driven for rotation by the electric motor, the nut cooperating with an axially moveable, non-rotating shaft by way of a ball-screw or similar low friction coupling.
There is a risk that the reaction forces experienced by the rack as a result of its cooperation with the pinion could result in significant side loadings being transmitted to the shaft of the actuator as the cooperation between the rack and the pinion results in the rack experiencing significant reaction loadings in directions angled to the longitudinal axis of the rack. The application of such loadings could result in increased wear or damage to the actuator which, clearly, is undesirable.
Whilst electrically driven linear actuators of the type described hereinbefore are capable of applying appropriately high axial forces to allow their use in a wide range of aerospace applications, they may be subject to increased levels of wear where significant side loadings are applied thereto, in use, such as where used to drive the rack of a nose wheel steering arrangement. In order to avoid the application of such side loadings, the support of the steering rack may be enhanced to permit reaction of the side loadings to an associated support. However, this may result in increased weight, and may also require the use of a more powerful actuator to overcome, for example, increased frictional loadings.
EP1548327 describes an electrically powered actuator of the general type described hereinbefore and in which the screw shaft is carried upon a support, a roller arrangement being provided between the screw shaft and the support to allow the screw shaft to move axially along the support but to substantially prevent angular movement therebetween.
It is an object of the invention to provide an actuator arrangement suitable for use in applications in which significant side loadings may be experienced, for example in nose wheel steering applications.